1. Field of the Invention
This invention relates generally to printing or recording devices that deposit ink on a recording medium, such as paper. More specifically, the invention relates to such devices wherein a rotating cylindrical drum moves a sheet of paper beneath an electrically driven and piezoelectrically actuated ink ejection mechanism. In particular, the invention relates to the aspects of controlling the rotation of the drum and to the subsequent timing of the ejection of the ink.
2. Description of the Prior Art
Good descriptions of the nature of one type of piezoelectrically actuated ink jet print head are contained in the Stemme and Mizoguchi patents mentioned above. An explanation of one way to control such a print head in the context of operation in a facsimile device is contained in the Hayami patent. The essential teachings of those patents are as follows.
The mechanical motion available from an electrically driven piezoelectric crystal can be used to move a diaphram and force a small amount of the ink in a chamber through a small orifice, so that the ejected ink impinges onto a sheet of paper in the vicinity of the orifice. The geometery of the chamber, the size of the orifice and the nature of the crystal can be chosen so that exactly one drop of ink is ejected for each cycle of mechanical movement of the crystal. In the absence of excitation of the crystal, surface tension keeps the ink from leaking out through the orifice.
A problem associated with printers of this type concerns the generation of spurious ink drops due to unwanted resonances of the ink in the chamber. The resonances are aggravated both by various data patterns and by increases to the basic drop generation rate. The effect of these spurious drops is to visibly reduce the quality of the printed image. The Hayami patent is an attempt to deal with this problem by continuously exciting the print head at a level below a threshold for ejecting drops, and using a synchronized increase in excitation amplitude to eject a drop. One drawback to this scheme is the requirement that the generation of the drop be synchronized with an oscillator through the action of a zero crossing detector. Possible variations in drum speed are not accounted for, perhaps because those inventors ensured that there was no significant variation in drum speed.
For reasons mentioned below, it was desirable to employ a print head control scheme that is tolerant of continuous variation in the angular velocity of the drum. The control scheme of the Hayami patent was tried, first without any attempt to account for variations in drum speed. While the print head itself performed well, the placement of the drops on the paper was totally unacceptable.
Next, the Hayami scheme was modified in the following way. The frequency of the continuous low level excitation was made to track the variations in the angular velocity of the drum. Whenever drops were required amplitude modulation at the zero crossing points was used to raise the excitation to levels sufficient to eject ink. This scheme corrected the positional accuracy of drop placement, but the quality of the drops (size, tendancy towards spurious drop generation) was found to be very difficult to control. The basic reason for this difficulty is connected with the fact that the excitation threshold for drop ejection and the optimum value of excitation for drop ejection both vary as a function of excitation frequency. It was exactly that frequency, however, that was made to vary as a function of variations in the angular velocity of the rotating cylindrical drum.
The result of those experiments was to abandon attempts to use continuous low level excitation with amplitude modulation. It was too difficult to sufficiently control to avoid significant degradation in the quality of the printed result, given the continuous variations in the angular velocity of the drum.